The present invention relates to a clutch adapted to a motor driven antenna for automobiles and, more specifically, to a clutch for a motor driven antenna which obtains a clutch force by an operation of fitting/separating a projection into/from a recess formed on the pressure surface of two rotating bodies.
In a conventional clutch of this type, when a power supply is turned on, a motor rotates forward or backward, while a driving clutch plate on the motor side rotates. Accordingly a driven clutch plate pressed on the driving clutch plate rotates. If the driven clutch plate rotates, its rotating force is transmitted to an antenna element through an antenna extending/retracting rope (e.g., a rope with a rack) to extend or retract the antenna element.
The angular displacement of recesses, which are formed at regular intervals along a rotating track on the pressure surface of the driving clutch plate, and that of projections (each elastically holding, e.g., fitting balls), which are also formed at regular intervals along a rotating track on the pressure surface of the driven clutch plate, are usually inconsistent with each other in the initial state. For this reason, if a sliding frictional force is varied between the driving and driven clutch plates due to a vibration caused during the extension and retraction of the antenna element or a load is changed due to variations in sliding frictional resistance of the antenna element, the projections of the driven clutch plate, which contact the inner surface of the driving clutch plate, slip and their contact positions are shifted. If the projections of the driven clutch plate drop into the recesses of the driving clutch plate, both the clutch plates are fitted into each other to generate a relatively loud noise.
When the extending or retracting operation of the antenna element is completed, the driven clutch plate coupled to the antenna element stops rotating, whereas the driving clutch plate continues to rotate as long as the motor rotates. The clutch is therefore disengaged, and the driving clutch plate and driven clutch plate slip relative to each other. A fitting/separating operation between the recesses and projections on the pressure surfaces of both the clutch plates, is repeated. Thus, the antenna element is completely extended, and a bad effect due to locking of the motor is avoided. In the meantime, a loud noise is continuously generated by the fitting/separating operation.
In the conventional clutch for a motor driven antenna, the projections of the driven clutch plate are dropped into the recesses of the driving clutch plate to generate a loud noise during the extending and retracting operations of the antenna element. This noise is relatively less than that continuously caused when the extending or retracting operation is finished. Since, however, the timing at which the loud noise is generated is irregular, the noise considerably offends a user's ear, and the user may think that any trouble has been caused in the antenna